1. Field of the Invention
The present invention relates to an engine control apparatus for controlling an engine by performing drive control of an engine output adjusting member, such as a throttle valve, through an electrical actuator, such as a DC servo motor. More particularly, the invention relates to an engine control apparatus which can prevent or suppress resonance of an output adjusting member when the drive control is realized by feedback control.
2. Description of the Related Art
In an engine for a vehicle, an output adjusting member, such as a throttle valve, is mechanically coupled to an accelerator pedal through a wire. In the mechanically coupled throttle valve, a throttle position, i.e., a throttle valve opening or a rack position is uniquely determined by a depression amount, i.e., an operation amount of the accelerator pedal. However, in control wherein the accelerator operation amount and the throttle position have a unique relationship, i.e., a one-to-one correspondence, only an output corresponding to a depression amount of the accelerator pedal can be obtained when a high output is required, i.e., in an acceleration state. Therefore, it is difficult to accurately correspond to various travel state requests.
Thus, various proposals have been conventionally made. In these proposals, an accelerator pedal and a throttle valve or the like are coupled through an electrical actuator, drive characteristics of the actuator are varied in accordance with a vehicle travel request from a driver (or operator), and the actuator is operated according to the various characteristics, thereby realizing engine control according to a driver's will. For example, in a vehicle accelerator control apparatus described in Japanese Patent Laid-Open Publication (Kokai) No. 60-198343, the drive characteristics of the actuator are defined as a function between the accelerator operation amount and the throttle valve opening. A plurality of the function characteristics are prepared, one of the characteristics is selected on the basis of an accelerator operation speed, and a target throttle valve opening is determined by the selected function characteristic.
In an engine control apparatus for performing feedback control of a throttle valve using an electrical actuator, e.g., a DC servo motor, a direct drive system is normally employed, i.e., the drive shaft of the motor is directly coupled to the throttle valve. However, when the motor is directly mounted, a mounting position is limited in view of a space factor around the throttle valve, and this cannot be easily achieved. Even if it can be mounted, a heavy article, i.e., the motor acts on an intake manifold as a moment, and causes a problem of durability of the intake manifold. For this reason, use of the direct drive system which leads to a very large or heavy intake system around the throttle valve is preferably avoided.
Thus, the motor is mounted on a vehicle body, and the motor and the throttle valve are linked through a wire, so that the intake system around the throttle valve can be made compact. In this wire link system, since the wire as an intermediate member is interposed between the motor and the throttle valve, feedback control may be performed on the basis of position data of the throttle valve (or position data of the motor), i.e., so-called position feedback control.
However, the present inventor found that when feedback control for driving the throttle valve or the like through the wire was performed, the throttle valve was sometimes resonated. According to the measurement results of the present inventor, an oscillation range of the throttle valve due to resonance reached .+-.10.degree. in the worst case.